PSU passed paper clip test so....what to test next?

Have searched the forums and got some ideas but...My son's Maxdata pc with an Asus motherboard just stopped working. I have replaced the PSU but the old PSU passed the paper clip test when I got it out (as does the new one!). Where next? How do you test the on/ off button on the comp - and I have also read a dead CMOS battery might be the problem- is that true? Memory and graphics cards appear to be seated fine. What to do next please!

Success! I decided that there must be a conflict between the hard drives so I started to unplug them one at a time. It booted up into Windows after removing the first drive. So I have been installing updates and running malware checks and updating drivers. Will leave my son to sort out the drive conflict problem

Success! I decided that there must be a conflict between the hard drives so I started to unplug them one at a time. It booted up into Windows after removing the first drive. So I have been installing updates and running malware checks and updating drivers. Will leave my son to sort out the drive conflict problem

Success! I decided that there must be a conflict between the hard drives so I started to unplug them one at a time. It booted up into Windows after removing the first drive. So I have been installing updates and running malware checks and updating drivers. Will leave my son to sort out the drive conflict problem